Impact of snow fence construction on tundra soil temperatures at Barrow, Alaska

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1 Permafrost, Phillips, Springman & Arenson (eds) 2003 Swets & Zeitlinger, Lisse, ISBN Impact of snow fence construction on tundra soil temperatures at Barrow, Alaska K.M. Hinkel Department of Geography, University of Cincinnati, Cincinnati J.G. Bockheim Department of Soil Science, University of Wisconsin, Madison K.M. Peterson Department of Biological Sciences, University of Alaska, Anchorage D.W. Norton Arctic Rim Research, Fairbanks ABSTRACT: In autumn 1997 a snow fence was constructed near Barrow. The wooden plank structure is 4 m high and extends 3.2 km. In October 1998, soil temperature loggers were installed at depths of 5 and 25 cm at several sites near the snow fence and in an unaffected control region. Snow thickness is measured along surveyed transects each spring, and thaw depth is measured along the same transects in August. Typically, the drift exceeds 4 m in height and extends 50 m downwind from the fence for nearly its entire length. Near-surface soil temperature beneath the drift averages 8 C warmer during the winter compared to the control. Soil in scour zones begins to thaw in late May owing to the thin or absent snow cover, and experiences relatively deep summer thaw. Thaw depth beneath the drift is relatively shallow because the drift persists 4 10 weeks, thus delaying soil thaw initiation. There is some evidence to suggest ground subsidence of the ice-rich permafrost is occurring, but the effects of thaw consolidation cannot be detected by surface probing. A program of high-resolution surveying, using differential GPS methods, is being implemented. 1 INTRODUCTION In autumn 1997, a snow fence was installed east of the village of Barrow (71 20 N, W). Residents in the newer development of Browerville had complained of deep and extensive drifting that often covered their doors and windows. The snow fence is situated about 1 km upwind from the development and is designed to capture snow driven by the prevailing easterly winter winds. The wooden plank structure is about 4 m high and extends approximately 3.2 km from Middle Salt Lagoon south to near Footprint Lake. It is located about 200 m east of Cakeater Road (Fig. 1). Anecdotal information suggests that a large drift formed just downwind of the snow fence in winter The lee drift extended 100 m from the fence for nearly its entire length and, in some places, reached the height of the fence. A smaller drift also developed upwind of the fence. The lee drift was still present the following June, although the surrounding tundra was free of snow. The thick accumulation resisted melting and the drift persisted until mid-july. In August, it appeared that the tundra beneath the melted snowdrift was significantly wetter and greener than the surrounding unaffected tundra, and there were indications that ground subsidence had occurred (Fig. 2). To determine the impact of deep snow drifting on soil temperature, thermistors were installed at depths of 5 and 25 cm at several sites near the snow fence in Figure 1. View of snow fence and drift in August 2001, looking south. Middle Salt Lagoon in foreground. October Three sites were established along a DRIFT transect perpendicular to the snow fence and drift crest: near the base of the snow fence (1 m downwind) where scouring had been observed the previous winter, 27 m downwind where drift thickness was maximized, and 114 m downwind where a lee scour zone developed. Three additional temperature monitoring sites were established along an unaffected CONTROL transect 300 m south of the snow fence. Beginning on 10 October 1998, temperature was recorded every two hours and is plotted in Figure 3. Because the 401

2 temperature traces for the three CONTROL sites were very similar, the average for the three is plotted. Further, since the soil temperature at 25 cm is nearly identical to the temperature at the 5 cm depth during winter, only the upper trace is plotted. In mid-may 1999, snow thickness was measured at 5 m intervals along the DRIFT and CONTROL transects. The drift crest exceeded 4 m in height, while the average snow thickness in undisturbed tundra was about 40 cm. In mid-august 1999, thaw depth was measured at 2.5 m intervals along the same transects. These data are shown in Figure 4. Figure 2. View along the leeward drift crest in August 1999 showing ponds possibly caused by thaw subsidence. Temperature (C) Soil Temperature at (5 cm) Control Fence Scour (1 m) Drift Crest (27 m) Lee Scour (114 m) 10-Oct 19-Nov 29-Dec 7-Feb 19-Mar 28-Apr 7-Jun 17-Jul Figure 3. Soil temperature at 5 cm depth measured every 2 hr beginning 10 October The trace labeled CON- TROL is the average of measurements made at the three control sites. 2 SITE DESCRIPTION Barrow has a cold maritime climate. Winters are long, dry, and cold, and summers are short, moist, and cool. The mean annual air temperature is 12.6 C; July is the warmest month at 4.1 C, and February is the coldest at 27.7 C (NOAA 1996). Mean annual precipitation is 124 mm, 37% of which falls as rain during July and August. Five major land-cover types are recognized in the Barrow region; these include dry heath, dry meadow, moist meadow, wet meadow, and emergent aquatic tundra vegetations ( ikonoslandcover.html). Soils of the Barrow region are underlain by permafrost within 0.5 m of the surface, and are classified as Gelisols. Three suborders occur in the region. Turbels, or soils affected by frost mixing (cryoturbation), comprise 77% of the soils mapped. Orthels are mineral soils unaffected by cryoturbation and account for 9% of the area, while organic soils (Histels) make up about 1%. Modern beach sediments, considered a miscellaneous land type, cover 4% of the area and water an additional 9%. Typic Aquiturbels are the dominant soil subgroup, covering 55% of the area. Snow Depth (cm) Drift Transect 14 May 1999 Thaw Depth: 17 Aug 1999 Fence Distance from Fence (m) Figure 4. Snow thickness and thaw depth (cm) near snow fence. Open circles indicate thaw depth measured beneath standing water; missing circles are used where deep water prohibited measurement. East (upwind) is to the left. 3 SIGNIFICANCE AND POTENTIAL IMPACTS Increasing local snow cover thickness can have several effects. First, since snow is an effective thermal insulator, ground heat loss in winter is reduced and soil temperatures are not as cold. As a result, soil warming and thaw in summer requires less energy as the system enthalpy is higher, and more heat energy is available to warm the soil. Thus, there is an overall tendency for the average annual soil temperature to increase. In regions underlain by permafrost, the active layer would become deeper as the upper permafrost melts. Melting of supersaturated permafrost results in ground subsidence and thermokarst. 402

3 Field evidence supporting this scenario was reported by Nicholson (1978) for the Schefferville mining area (55 N) in the center of the Nouveau-Quebec-Labrador Peninsula. In an effort to cause permafrost degradation and increase the thickness of the active layer, snow fences were installed that increased snow cover thickness from 10 to 90 cm. Over a 5-year period, the active layer progressively deepened; by the end of the experiment it was 2.5 times (6.5 m) deeper than the control. The effect was largely attributable to warmer soil conditions in winter. However, there is some evidence to indicate that drifting can also counteract soil warming (e.g. Walker et al. 2001). Thick accumulations of snow persist several weeks or months after the surrounding tundra is snow free. Thus, soil warming can be delayed and active layer thickness reduced. Resolving these opposing effects requires examination of the direct and indirect impact of deep snow drifting on the soil thermal regime. For example, snow accumulation influences the transient properties of the underlying soil. The melting snow promotes water ponding at the ground surface and enhances meltwater infiltration into the ground. Previous research (Hinkel et al. 1993, 1997, 2001, Kane et al. 2001) has shown that meltwater percolating into the frozen ground through pores and contraction microcracks causes rapid warming of the near-surface soil. In addition, as air is displaced by water in soil pores, the soil thermal conductivity increases and the potential downward soil heat flux is enhanced (Farouki 1981). To date, there is no general agreement on the longterm influence of deep snow drifting on the ground thermal regime. Our experience at sites on the North Slope of Alaska suggests that deep drifting tends to promote melting of the upper permafrost in regions where the substrate is ice rich, resulting in ground subsidence and thermokarst. The snow fence at Barrow provides an opportunity to monitor the soil thermal impact from the inception of the disturbance. Since the silty substrate around Barrow is extremely ice-rich, exceeding 70% in the upper 2 m (Brown & Johnson 1965), the ground is highly susceptible to thaw subsidence. Furthermore, a change in the soil moisture regime and plant community are likely outcomes over a fairly short time period (Scott & Rouse 1995). 4 SOIL TEMPERATURE RECORDS The average temperature trace at the 5 cm depth for the three CONTROL plots is shown in Figure 3. Minimum temperatures of around 20 C occur in February, and thaw begins in mid-june. Significant damping of the thermal signal is observed, but the effects of synoptic events are apparent. Soil temperature beneath the DRIFT crest (27 m) reached a minimum of 8 C; this occurred in early June. The minima is followed by a very rapid increase in soil temperature beginning 10 June, from 6.27 to 1.56 C ( 4.71 C) in less than two days. This rate of change far exceeds the average, and likely reflects the thermal impact of snow meltwater infiltration. Soil temperature gradually warmed toward 0 C throughout the summer, but soil thaw did not begin until the drift ablated in mid-july. Synoptic effects are absent from the winter record. Conversely, the fence and lee scour zones experienced temperatures colder than 30 C in February. Furthermore, the high-frequency variations in the temperature traces indicate little thermal damping. Separation of the scour traces in early winter is due to the difference in the snow thickness at these sites. Wind scouring and soil exposure results in very similar temperature traces after March. Thaw began at the snow fence scour zone around 1 June; the record for the lee scour zone (114 m) ends at this time due to vandalism. The thermal impact of the snow can be quantified by subtracting the mean daily temperature (MDT) at the 5 cm level at each DRIFT site from the MDT of the CONTROL; positive values indicate that the soil beneath the drift is warmer. Figure 5 suggests that drifting began near the end of November when soil temperature beneath the drift is substantially warmer ( 10 C) than the control; this is confirmed by National Weather Service data indicating a blizzard at this time. Over the period October June, the soil temperature beneath the drift was, on average, 8 C warmer than the soil along the control. Temperature (C) Temperature Difference Using MDT of Control Fence scour (1 m) minus Control Drift crest (27 m) minus Control Lee scour (114 m) minus Control Soil colder than control Soil warmer than control 10-Oct 19-Nov 29-Dec 7-Feb 19-Mar 28-Apr 7-Jun 17-Jul Figure 5. Temperature residuals using mean daily temperature (MDT) of the CONTROL at the 5 cm level; positive values indicate that the soil beneath the drift is warmer. 403

4 Scouring beneath the fence resulted in substantially colder ( 3.4 C) soil temperatures, which persisted throughout the winter. The lee scour zone demonstrates drifting effects, following by scouring in late April. 5 PATTERNS OF GROUND THAW The pattern of thaw depth collected in mid-august (Fig. 4) indicates relatively shallow thaw beneath the snow drift and enhanced thaw in the fence and lee scour zones. This implies that heavy drifting stabilizes permafrost by reducing the length of the thaw season, while scouring promotes deeper thaw and permafrost melting. As shown in Figure 2, the area affected by drifting appears very wet. Pools were too deep in several places to collect thaw depth measurements. In some locations, standing water may be due to destruction of the protective organic mat during fence construction. However, given the preferential occurrence of saturated soil conditions beneath the drift crest along most of the fence length, it is possible that permafrost melting and subsidence has occurred. Since surface probing cannot detect subsidence effects, accurate surveying using differential GPS in subsequent summers will be used to monitor ground subsidence in the control plots. 6 ONGOING STUDIES In summer 1999, additional data loggers were deployed to collect soil temperature measurements at depths of 5, 30, and 50 cm from twelve sites affected by drifting. Data loggers, powered by long-life batteries, were placed in waterproof containers and buried to prevent further damage by vandalism. Measurements of soil moisture were also collected to determine if the soil is becoming wetter. An extensive program of soil and vegetation sampling and mapping was conducted near the fence and in the control region to monitor the alteration of soil properties and plant communities in response to changes in the soil temperature and moisture regimes (Scott & Rouse 1995). Soil temperature measurements have been collected continuously since that time, although the temperature data has not been downloaded since logger installation. Early snowfall during field visits effectively concealed logger location in autumn The snow drift that developed in winter was similar to that of the previous winter. However, winter was characterized by heavy snow and strong winds, and the drift that formed has covered logger containers since that time. By April 2001, the drift exceeded 4 m in height and had completely buried the snow fence along most of its length. Compared to previous years, it was substantially wider both upwind and downwind from the fence. The leeward drift contained about 60% more snow (by volume) than previous years although the snowcover on the tundra was thinner (32 cm) than previously measured at the control plots (Fig. 1). Replicate measurements collected in June 2001 indicated little drift ablation ( 50 cm) over the previous two months. The summer of 2001 was significantly cooler than normal; the average temperature for the period June- August was 0.3 C compared to 3.2 C in By mid-august, it became apparent that the drift remnant would persist throughout the summer. Infiltration of snow meltwater, ponding atop the frozen ground, and subsequent recrystallization produced a slab of ice averaging 0.5 m in thickness and about 50 m in width. Thus, it was not possible to recover the temperature data from most loggers. It will be interesting to see if the drift melts in subsequent years, or if changes to the microclimate result in the net accumulation of mass over time. There is some visual evidence that ground subsidence has occurred in recent years, and particularly in response to the formation of the large drift in The vertical wooden planks originally extended downward nearly to the tundra surface. By summer 2001, there was a gap below the planks of about 40 cm along most of the fence length. Similarly, the 25 cm diameter vertical fence support pipes show evidence of corrosion extending 40 cm above the current soil level, presumably due to contact with wet soil. It is therefore surmised that the ground has subsided about 40 cm since fence installation. Comparison with photographs taken soon after fence installation supports this interpretation. 7 CONCLUSIONS The drift developed primarily during blizzard events and exceeded 4 m in height, while the average snow thickness in undisturbed tundra was about 40 cm. Soil temperatures beneath the drift averaged about 8 C warmer during the winter compared to the control, while soil temperature in the scour zones were about 3 C colder. The scour zones began to thaw in late May owing to the thin or absent snow cover, and experienced relatively deep summer thaw. Thaw depth beneath the drift was relatively shallow because the drift persisted until mid July. However, measuring thaw depth by surface probing can yield misleading results since this method cannot detect the effects of thaw consolidation in ice-rich permafrost. High precision surveying, using differential GPS methods, is currently underway to monitor ground subsidence in the area immediately affected by drifting and scoring. 404

5 ACKNOWLEDGEMENTS This research was supported by grants from the National Science Foundation to KMH (OPP , , and ). Any opinions, findings, conclusions, or recommendations expressed in the material are those of the authors and do not necessarily reflect the views of the National Science Foundation. We are grateful to the Ukpeagvik Inupiat Corporation for access to the Barrow Environmental Observatory, and the Barrow Arctic Science Consortium for administrative and logistic assistance. REFERENCES Brown, J. & Johnson, P.L Pedo-ecological investigations at Barrow, Alaska. CRREL Technical Report 159. Farouki, O.T Thermal Properties of Soils. CRREL Monograph Hinkel, K.M., Outcalt, S.I. & Nelson, F.E Near-surface summer heat-transfer regimes at adjacent permafrost and non-permafrost sites in central Alaska. In Proceedings of the Sixth International Conference on Permafrost. South China University of Technology Press: Hinkel, K.M., Outcalt, S.I. & Taylor, A.E Seasonal patterns of coupled flow in the active layer at three sites in northwest North America. Canadian Journal of Earth Sciences 34: Hinkel, K.M., Paetzold, R.F., Nelson, F.E. & Bockheim, J.G Patterns of soil temperature and moisture in the active layer and upper permafrost at Barrow, Alaska: Global and Planetary Change 29: Kane, D.L., Hinkel, K.M., Goering, D.J., Hinzman, L.D. & Outcalt, S.I Nonconductive heat transfer associated with freezing soils. Global and Planetary Change 29: National Oceanic and Atmospheric Administration Nicholson, F.H Permafrost modification by changing the natural energy budget. In Proceedings, Third International Conference on Permafrost. National Research Council of Canada, Vol. 1: Scott, P.A. & Rouse, W.R Impacts of increased winter snow cover on upland tundra vegetation: a case example. Climate Research 5: Walker, D.A., Billings, W.D. & De Molenaar, J.G Snow-vegetation interactions in tundra environments. In H.G. Jones, J.W. Pomeroy, D.A. Walker & R.W. Hoham (eds), Snow Ecology: An Interdisciplinary Examination of Snow-Covered Ecosystems: Cambridge: Cambridge University Press. 405

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